This invention relates to a system for racking drill pipes, used in the drilling activity and, more in particular to a method where the drill pipes, in stands averaging usually 90 feet in length, are positioned vertically from the rig floor downward into a container located in the area of the rig substructure and, to the related apparatus necessary to handle the stands of pipes from the centerline of the Rotary Table to the racking area position and, viceversa.
In the drilling of a well, anytime the drilling bit is worn-out, it is necessary to extract from the hole the whole string of drill pipes to replace the worn-out bit and then to lower it back into the same hole to resume drilling operation. In doing this, it is customary, as well known in the drilling art, to break down the drill string into stands, usually 90 feet long.
Today, in all fixed type rigs and also in the largest percentage of the floater type platform, the stands are racked by man power inside the derrick, in vertically standing position from the rig floor upward.
Two types of mechanical racking systems are today available for the floater type platforms:
ONE TYPE IS A SYSTEM HAVING BASICALLY THREE HORIZONTAL ARMS MOUNTED AT DIFFERENT ELEVATIONS ON THE FRONT SIDE OF THE DERRICK AND ACCOMPLISHING A MECHANICAL HANDLING OF THE STAND FROM THE CENTERLINE OF THE Rotary Table to the setback area position on the rig floor and, viceversa; the other type of system allows for a horizontal racking of the stands on the deck of the ship and out of the derrick configuration, basically accomplished by mechanical handling devices, where the stand is first pulled out of the derrick and than racked horizontally.
In the first system, since the pipes are racked into the derrick, a stronger derrick is required to sustain the dynamic load deriving from the floater motion, and also floater stability may be quite compromised in severe sea conditions.
The second mentioned system is suitable for installation only on drilling ships: the derrick is not subject to dynamic forces and ship stability is improved as a result of the lower center of gravity of the racked pipes, but the bending damage occurring to the stands as a result of the transferring operation from the derrick to the ship deck and, viceversa, is also well recognized.